Extendable drill bit assemblies provide for use of a drill at an extended distance from the location to be drilled. These extendable drill bit assemblies include flexible rods. Alignment of the drill bit with the location to be drilled often results in a bend in the flexible rods allowing the operator to access locations for drilling which may otherwise be inaccessible to the operator. Rotational forces are transferred from the chuck of the drill to the drill bit including along the bend path. As a result of the bend in the path, forces occur between the head of the shank of the tail piece and the chuck of the drill and, at times, results in the head of the shank of the tail piece being sheared from the remainder of the tail piece.
U.S. Pat. No. 4,990,038 to DeLong discloses a drill bit which is received by a collet attached to the distal end of a flexible drive cable. The drill bit includes a boss at the proximal end thereof which aids in retaining the drill bit within the collet as the drill bit is removed from the material being drilled. The boss includes a stem and a head. In DeLong, the drill bit is assembled into the collet by sliding the drill bit drive end into the collet and radially manipulating until the boss head passes through an opening in a wall of the collet. Thereafter, the drill bit is rotated until the boss head is locked behind the wall. When locked in this manner, the drill bit cannot be removed by mere application of an axial withdrawal force. Also, rotation of the drill bit in the collet is restricted in angular extent by the wall so that the drill bit can be removed by merely rotating. In DeLong, although the dimension of the boss is reduced relative to the dimension of the shaft of the drill bit, due to the offset of the boss, no clearance is provided between an outer surface of the boss head and the inner surface of the wall of the collet. Thus, upon the application of rotational forces and upon flexing of the drive shaft, the boss will be subjected to forces which may cause the boss to shear. In addition, the boss is not required to retain the locking ball of a quick-change locking system in a locked position.
U.S. Pat. No. 5,167,476 to Lafferty et al. discloses a drill bit, which includes an eccentric extension. The eccentric extension is provided at the proximal end of the drill bit and matches in size and shape a recess of the collet. The drill bit is locked in rotational engagement with the collet by the eccentric extension engaging the recess. With the extension in the recess, there us no slippage of the drill bit in the collet. Although the eccentric extension is reduced in diameter relative to the shaft of the drill bit, the extension is offset. Thus, when positioned within the collet, the outer surface of the extension will abut the inner surface of the collet. Because no clearance is provided, upon rotation and flexing of the shaft, the forces applied to the extension may cause the extension to shear. In addition, the extension does not serve to retain a locking ball of a quick-change locking mechanism in the locked position.
U.S. Pat. No. 5,597,275 to Hogan discloses an adapter mounted to a drill. The adaptor includes a forward extending shaft which is cut away to form a tongue and a groove. The drill bit to be mounted to the drill also includes a tongue and groove which is designed to mate with the tongue and groove of the shaft. With this engagement, rotation of shaft is transmitted to the drill bit so that the two will rotate together and working tip (drill bit) is held against axial movement, i.e., movement forwardly or rearwardly with respect to shaft. Another embodiment in Hogan provides a shaft having an engagement end including a half round end portion which mates with a half round end portion of a drill bit. None of the embodiments disclosed in Hogan include a quick-change mechanism with a locking ball to be held in a locked position. In addition, none of the embodiments disclosed in Hogan include a tail piece having a reduced dimension which provide clearance between the outer surface of a proximal end of the tail piece and the quick change mechanism which receives the tail piece.
U.S. Pat. No. 4,850,758 to Morgan discloses a quick-change drill bit including a drill bit holder which receives a drill bit. The shaft of the drill bit is hexagonal in shape about 5/16″ across flats. This increased size prevents these drill bits from being used in currently manufactured holders without the safety provided by the split sleeves provided in Morgan. The increased size also overcomes the loss of strength caused by the circular groove. About ½″ of drill bit shaft fits into the holder, the inside of which is also hexagonal in shape and sized to fit the approximate 5/16″ size of the drill bit shaft. The drill bit shaft has a circular groove cut only into the high points of the hex-shape to engage a “C” ring inside the holder. The drill bit shaft has a circular groove to provide shoulders for the split sleeve ends to hold against. Although Morgan contemplates that the shaft of the drill bit will have a radial dimension smaller than the radial dimension of the working end of the drill bit, Morgan does not disclose a proximal end of a shaft having radial dimensions smaller than the inner dimension of the holder to provide clearance between the holder and the proximal end of the shaft. Rather, Morgan discusses increasing the radial dimensions at the proximal end of the shaft to avoid use in “currently manufactured” holders.
The prior art shows drill bits having drive shaft portions which are received by the chuck of the drill or an adaptor. The prior art also shows that the drive shaft portion may include a reduced dimensioned section, however, the reduced dimension sections of these drive shafts are not configured to provide clearance between the reduced dimension section and the chuck of the drill or the interior surface of the adaptor in which it is received. In addition, the prior art which provides for drive shafts having reduced dimensions are not configured to retain a locking mechanism of a quick-change assembly.
The present invention provides an improved tail piece for use in an extendable drill bit assembly which overcomes the problems presented in the prior art and which provides additional advantages over the prior art, such advantages will become clear upon a reading of the attached specification in combination with a study of the drawings.